The invention comes within the technical field of orthopaedic implants.
More particularly, the invention concerns arthroplasty of the shoulder joint, particularly treatment of centred glenohumeral osteoarthritis.
In a way well known to those working in the field, the surgeon implants a glenoid prosthesis to compensate for the wear in an arthritic glenoid.
Essentially, this type of prosthesis generally consists of either a cemented polyethylene cup or a non-cemented tray to receive a polyethylene cup which will be in contact with a prosthetic humeral head, with which it will articulate.
It has been found that this type of prosthesis has disadvantages given that there is a tendency for the joint to be lateralised, which as a consequence modifies the centres of rotation of the shoulder leading to a breakout torque between the point at which the effort is applied by the head of the humerus and its fixation on the glenoid. A lever arm results composed of the thickness of the polyethylene cup which is in the order of 6 mm when it is cemented and 8 to 9 mm when the cup is combined, as indicated, with a tray. This lever arm forms a not insignificant breakout torque which can lead to premature loosening.
Recent biomechanical studies have demonstrated that the instantaneous centre of rotation is situated within an area of 5 to 8 mm diameter and does not follow translation consistent with rotation of the joint, as earlier publications would have it and has been generally accepted by surgeons.
Based on this observation, the problem that the invention proposes to solve is simply that of replacing the cartilage, without modifying the biomechanical architecture.